OLD AGE AND SENESCENCE 



517 



much delayed in tethelin-fed animals, the loss of weight for a prolonged 

 period being almost imperceptibly gradual, whereas in normal animals 

 it is relatively sudden (Fig. 41, p. 506). 



We have seen that the tissues of the Nervous System are very rich 

 in lipoids which are either identical with (cholesterol) or related to 

 (phospholipins, etc.), the substances which we know to have an 

 influence upon growth similar to that which we would expect to be 

 exerted by catalyzers of growth. Furthermore their exceptionally 

 high Metabolic Rate encourages the supposition that they produce an 

 abundance of endogenous growth-catalyzers. A predominant develop- 

 ment of nervous tissues should therefore be equivalent in its effects 

 upon metabolism, growth, and life-duration to the continuous adminis- 

 tration of an excess of growth-catalyzers. 



Now Friedenthal has pointed out that the ratio of brain-weight to 

 body-weight or to the two-thirds power of the body-weight, which 

 he terms the " cephalization-f actor," varies from one species of animal 

 or bird to another in extremely close correspondence with the maximal 

 attainable duration of life. The following are among the figures which 

 he cites in support of this thesis: 1 



MAMMALS. 



Maximal life-duration 



Cephalization-factor. (according to Hanseman 



Species. in years. 



Man. . 2. 67 to 2. 81 80 to 150 



Elephant 1.24 to 1.34 90 to 100 



Anthropoid apes 0.76 to 0.65 



Horse I, ... 0.43 to 0.57 45 



Deer v . .. 0.40to0.50 30 



Bears 0.36 to 0.50 50 



Dogs 0.34 to 0.51 15 to 20 



Cats 0.29 to 0.34 20 



Oxen 1 



Giraffes > 0.30 to 0.40 30 



Antelopes J 



Squirrels 0.16 to 0.20 6 



Insectivora 0.06 to 0.18 6 to 10 



Mice ..." 0.04 3 



BIRDS. 



Maximal life-duration 

 (according to Hansemann 

 Species. Cephalization-factor. in years. 



Carrion crow 0.168 100 (?) 



Parrots 0.147 to 0.177 100 (?) 



Alpine crow 0.114 50 



Buzaard 0.11 



Owl 0.113 



Finch 0.086 8 



Sparrow 0.086 



Duck 0.0731 



Snipe 0.0585 



Quail 0.0495 



Heron . 0.0459 15 



Pheasant . 0.0343 15 



Fowls 0.0249 10 to 20 



Ostrich . 0. 0195 



1 The life-duration of the mouse computed from the observations cited above has 

 been added to the table. 



